1. Explain Negative pressure ventilation
2. Describe how the ventilator functions and how input power is converted to ventilator breaths, how breaths are controlled by the ventilator and the variables that describe the modes of ventilation
3. List and describe the three phase variables for a mechanical breath to be delivered: Trigger, Limit, and Cycle
Negative pressure ventilators were inspired in the early 1900’s, before the polio epidemic, to provide artificial respiration. The use of negative pressure ventilators today is rare but understanding their use helps provide a greater understanding of ventilators. Negative pressure ventilators were intended to mimic natural spontaneous breathing by creating pressure gradients around the patient’s chest. They provided noninvasive ventilation, which was accomplished by using either a shell that surrounded the chest or a cylinder that enclosed the entire body. The principle of these devices is that a vacuum pump created subatmospheric pressure (negative pressure) intermittently in a chamber surrounding the thorax (chest wall) which resulted in the expansion,of the patient’s chest initiating inspiration. When the vacuum was terminated, the negative pressure outside the chest wall returned to zero and the elastic recoil of the chest and lungs permitted passive exhalation allowing air to flow out of the lungs into the atmosphere. When negative pressure is being applied to the thorax air flows into the lungs from the atmosphere.
There are two primary types of negative pressure ventilators: Iron lung and chest cuirass. An iron lung is a device where the entire body, except for the head, is placed in a chamber with an airtight seal around the neck. A chest cuirass is a jacket with seals around the neck, arms, and thighs. The chest cuirass was suitable for home use and provided much more versatility than the iron lung.
Modern mechanical ventilators use positive pressure to provide ventilatory support. Positive-pressure ventilation is an invasive technique that uses an endotracheal or tracheostomy tube to push gas directly into a patient’s lungs until the machine terminates the breath. The pushing of gas into the patient’s lungs creates positive pressure as opposed to negative pressure, which is created by the downward pull of the diaphragm and the outward expansion of the chest wall creating pressure gradients and therefore causing air to flow into the lungs.
Input power is simply the power source that enables a device to function. The power source provides a ventilator with the energy to perform the work required to ventilate a patient. A power source may be an electric or gas source. Ventilators that are electrically powered are most commonly used in homes. Ventilators that use a 50psi gas source are referred to as pneumatically-powered and are most commonly used for transporting ventilator dependent patients throughout the hospital or during an MRI. Patients that use both electrical and pneumatic power are referred to as combined powered ventilators and are most commonly used in the Intensive Care Units (ICU).
It is important to note that when reference is made to a ventilator breath it implies that the inspiratory phase of a breath is delivered and exhalation is always passive. In order to understand mechanical ventilation one must understand the phases of inspiration during ventilation. There are three phases of inspiration. The beginning (how and when inspiration starts), the middle (how much air enters the lungs and how much the lungs expand), and the end (how and when inspiration ends). Mechanical ventilation uses ventilators to deliver gas to the lungs using positive pressure at a certain rate (respiratory rate). The amount of gas delivered can be limited by time, pressure, volume, or flow. The duration of inspiration can be cycled(terminated) by time or volume. There are three phase variables that define the parameters of a ventilator breath. The three parameters are set by the clinician on the machine and include: how the breath begins (trigger), how the breath is delivered (limit), and how the breath ends (cycle). Another equally important parameter that is also set by the clinician determines how often a breath is delivered (respiratory rate).
How a mechanical breath begins is defined by the trigger variable. The breath trigger variable is known as the sensitivity control. The limit variable places a maximum value or limit on a control variable during delivery of a breath. During mechanical ventilation of an adult the limit or control variable can be pressure limited or volume limited. A limit variable is a parameter that rises to a certain value but does not exceed it. For example, a patient with ARDS has extremely non-compliant lungs (stiff lungs). When ventilating these patients, care must be taken during mechanical ventilation to not over distend the lungs, which can result in a pneumothorax and further damage to the lungs. These patients are placed on pressure control also known as pressure limited ventilation and a pressure level is set. Once the breath is triggered (begins) then the machine will deliver pressure and the pressure will rise until it reaches the preset level. Once the preset pressure level is reached then it is maintained at that level until inspiration ends. The cycle variable is the parameter that ends the inspiratory phase and allows passive exhalation to begin. During pressure control ventilation the cycle variable is time. An inspiratory time (I time) is set and once the set time elapses then inspiration ends. During volume control ventilation the amount of air to be delivered during inspiration (tidal volume) is set and when the set volume is reached inspiration ends. Therefore, it is important to remember that pressure control/pressure limited ventilation is time cycled and volume control/volume limited ventilation is volume cycled.
There are three primary types of breaths delivered by a ventilator: mandatory, assisted, and spontaneous. A mandatory breath indicates that the ventilator does all the work of breathing and the patient does none of the work required to breathe. This type of breath allows the patient to fully rest their inspiratory muscles. During a mandatory breath all three phases of inspiration are controlled. The ventilator starts the breath, controls inspiratory gas delivery and determines when inspiration ends. An assisted breath is a breath in which the patient does some of the work, however, the patient is only allowed to start or initiate a breath. Once inspiration begins the ventilator then takes over and controls the inspiratory phase and determines when inspiration ends. Therefore, during an assisted breath only 2 of the 3 phases of inspiration is controlled. A spontaneous breath is a breath in which the patient controls all phases of inspiration which means that the patient does all the work required to breathe. The ventilator monitors the patient parameters, notifies clinicians of issues through set alarms and serves as a backup if the need arises.
For this assignment, you will provide detailed responses to the following questions
1. Discuss the effects of positive pressure ventilation on oxygenation and ventilation
2. Define the following terms and explain their significance
· Peak inspiratory Pressure (PIP)
· Plateau pressure (Pplat)
· Airway resistance (Raw)
· Dynamic Compliance (Cdyn)
· Static Compliance (Cstat)
3. Define each of the ventilator modes listed below:(use your own words)
· Continuous Mandatory Ventilation
· Synchronized Intermittent Mandatory Ventilation
· Pressure Support Ventilation
· Continuous Positive Airway Pressure
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